A local seismic network, operating in western Argentina from 28°S to 33 °S, between August 1995 and January 1999, recorded more than 450 earthquakes. In this region, the geodynamic is controlled by the subduction of the Nazca plate beneath the South American lithosphere, which is characterised here by a sub-horizontal path before reassuming its downward descent. As accurate earthquake locations are of primary importance when studying the seismicity of a given area, events recorded by the local seismic network enable in-depth investigations into seismotectonic patterns, allowing us to improve the earthquake source characterization and the knowledge on the ongoing seismo-tectonics of the region. With this aim, we performed a simultaneous inversion of both the velocity structure and the hypocentre location. The obtained velocity images highlight vertical and lateral heterogeneities which, together with the distribution of the relocated events, can be associated with the main tectonic structures in the crust of the western Sierra Pampeanas region. The study also provided new constraints on the geometry of the subducted slab. We noted a great concentration of shallower seismicity with respect to that of the surrounding areas at the expected position of the Juan Fernandez Ridge, which certainly influences the subduction style along its strike, leading to the formation of a bent in the slab geometry.